<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    手机号：<input type="number">
    <button>发送</button>
    <script>
        var btn = document.querySelector("button")
        var time = 5
        btn.onclick = function()
        {
            btn.disabled = true;
            var id = setInterval(function(){
            time--
            btn.innerHTML="还剩下" + time + "秒"
            if(time==0)
            {
                clearInterval(id)
                btn.disabled = false
                btn.innerHTML = "发送"
            }
        },1000)
    }
    </script>
</body>
</html>